Interfacial thermal transfer structure

ABSTRACT

An apparatus that interfaces thermal transfer components is described. The apparatus includes a soft, thermally conductive metal that enables a capillary flow path with a contact surface of a thermal transfer component and an imbibing thermal interface material. The thermal transfer component is a heat sink. The thermally conductive metal includes large pores that intertwine with smaller pores of the contact surface.

TECHNICAL FIELD

The present techniques relate generally to the thermal management of acomputing system. More specifically, the present techniques relate tothe thermal management of a computing system using an interfacialthermal transfer structure.

BACKGROUND ART

Modern computer components generate large amounts of thermal energyduring operation. Such thermal energy negatively impacts the performanceof the components and results in heat related damage to the componentcomponents. Therefore, heat sinks are typically implemented to removethermal energy from components. Such heat sinks generally function, atleast in part, by thermal conduction through physical contact with aportion of the component.

Resistance to thermal conduction at an interface between a component anda heat sink can undermine the efficiency and effectiveness of the heatsink. Therefore, numerous thermal interface materials (TIMs) have beendeveloped to more efficiently conduct heat from the component to theheat sink. However, conventional TIMs, such as particle laden polymers,phase change materials, thermal pastes, and the like, are not veryreliable. A number of commercially-available TIMs have high initialperformance, but fail to meet end of life (EOL) requirements. TIMdegradation is exacerbated by several factors, including largeintegrated heat spreader (IHS) area, low compression pressure for TIMs,and flatness variation (or non-coplanarity).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 1B is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 2A is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 2B is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 3A is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 3B is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 4 is a block diagram showing an interfacial thermal transferstructure integrated to a heat sink;

FIG. 5 is a block diagram showing an embodiment of the interfacialthermal transfer structure;

FIG. 6A is a block diagram showing an example material of theinterfacial thermal transfer structure;

FIG. 6B is a block diagram showing the example material of theinterfacial thermal transfer structure; and

FIG. 7 is a computing device including an interfacial thermal transferstructure.

The same numbers are used throughout the disclosure and the figures toreference like components and features. Numbers in the 100 series referto features originally found in FIG. 1; numbers in the 200 series referto features originally found in FIG. 2; and so on.

DESCRIPTION OF THE EMBODIMENTS

Thermal solution designs often employ thermal pastes, phase changematerials, and the like to enable temperature control of lidded packagesto meet EOL reliability requirements. Often, these TIMs are composed ofa low thermal conductivity organic phase, such as silicone grease,interspersed with high conductivity metal or ceramic particles to enablea higher effective thermal conductivity composite. However, suchmaterials have several limitations, including large thermal resistances,susceptibility to voiding, and dry out (or pump out).

Increasing the volume fraction of particles increases thermalconductivity and effective viscosity of the material system. Thisincreased effective viscosity prevents the formation of thin bond lines,and reduces the contact resistance between the metal substrate (e.g.,the heat sink base or IHS) and the TIM. However, thermal pastesdispersed with high concentrations of particles suffer from thermalproperty variation because of flocculation of particles, particle-fluidphase separation during squeeze out, and high pressure generation at thesubstrate-particle contact points.

Reliability requirements dictate sustenance of minimum bond linethicknesses (BLTs) for TIMs throughout the life of the product. However,a known failure mechanism of wetting certain TIMs, such as thermalpastes, is void formation via paste pumping during repeated thermal andpressure cycles.

Accordingly, embodiments described herein provide an interfacial thermaltransfer structure to be used in association with conventional TIMs. Thestructure, in one embodiment, is a capillary-wick-enabled TIM thatcreates liquid wicking paths for conventional TIMs, and provides animproved thermal conductivity. An apparatus with thecapillary-wick-enabled TIM has improved thermal conductivity, incomparison to conventional TIMs, used without the interfacial thermaltransfer structure. Additionally, the capillary-wick-enabled TIM slowsthe degradation of conventional TIMs from pump-out.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.Rather, in particular embodiments, “connected” may be used to indicatethat two or more elements are in direct physical or electrical contactwith each other. “Coupled” may mean that two or more elements are indirect physical or electrical contact. However, “coupled” may also meanthat two or more elements are not in direct contact with each other, butyet still co-operate or interact with each other.

Some embodiments may be implemented in one or a combination of hardware,firmware, and software. Some embodiments may also be implemented asinstructions stored on a machine-readable medium, which may be read andexecuted by a computing platform to perform the operations describedherein. A machine-readable medium may include any mechanism for storingor transmitting information in a form readable by a machine, e.g., acomputer. For example, a machine-readable medium may include read onlymemory (ROM); random access memory (RAM); magnetic disk storage media;optical storage media; flash memory devices; or electrical, optical,acoustical or other form of propagated signals, e.g., carrier waves,infrared signals, digital signals, or the interfaces that transmitand/or receive signals, among others.

An embodiment is an implementation or example. Reference in thespecification to “an embodiment,” “one embodiment,” “some embodiments,”“various embodiments,” or “other embodiments” means that a particularfeature, structure, or characteristic described in connection with theembodiments is included in at least some embodiments, but notnecessarily all embodiments, of the present techniques. The variousappearances of “an embodiment,” “one embodiment,” or “some embodiments”are not necessarily all referring to the same embodiments. Elements oraspects from an embodiment can be combined with elements or aspects ofanother embodiment.

Not all components, features, structures, characteristics, etc.described and illustrated herein need be included in a particularembodiment or embodiments. If the specification states a component,feature, structure, or characteristic “may”, “might”, “can” or “could”be included, for example, that particular component, feature, structure,or characteristic is not required to be included. If the specificationor claim refers to “a” or “an” element, that does not mean there is onlyone of the element. If the specification or claims refer to “anadditional” element, that does not preclude there being more than one ofthe additional element.

It is to be noted that, although some embodiments have been described inreference to particular implementations, other implementations arepossible according to some embodiments. Additionally, the arrangementand/or order of circuit elements or other features illustrated in thedrawings and/or described herein need not be arranged in the particularway illustrated and described. Many other arrangements are possibleaccording to some embodiments.

In each system shown in a figure, the elements in some cases may eachhave a same reference number or a different reference number to suggestthat the elements represented could be different and/or similar.However, an element may be flexible enough to have differentimplementations and work with some or all of the systems shown ordescribed herein. The various elements shown in the figures may be thesame or different. Which one is referred to as a first element and whichis called a second element is arbitrary.

FIG. 1A is a block diagram showing an interfacial thermal transferstructure 102 integrated to a heat sink 104. The structure 102 is alsoreferred to herein as, the TIM carrier, and the continuous phase of theTIM. The continuous phase of the TIM may be created using a soft,thermally conductive material, e.g., polymers, metals, that enables acapillary flow path (smaller pores 106˜20-100 μm) intertwined with largepores 108 (˜1-2 mm). The TIM carrier may be constructed of expandedmetal grids that facilitate the creation of a dual-porosity wick, suchas a woven wire mesh. These wicking materials can be directly bonded tothe base of the heat sink 104 using conventional manufacturing processessuch as soldering, brazing, diffusing bonding, and the like.Alternatively, the wicking patterns can be machined on the base of theheat sink itself. The thickness of the TIM carrier may range from 25-500μm. The TIM carrier not only enables liquid wicking paths but alsoenables contact between top and bottom substrates, e.g., the heat sink104 and a the source (not shown), such as a circuit, controller, heatspreader, and the like. In the interstitial space 106 within the TIMcarrier, conventional TIMs, such as thermal pastes and greases can beplaced to improve the effective thermal conductivity of the interfacialthermal transfer structure 102.

Also, the interfacial thermal transfer structure 102 can manage multiplephases. Liquid imbibes in to smaller pores 106 between the TIM carrierand heatsink base 104 more rapidly, and to a higher level than, liquidimbibes into larger pores due to capillarity, which is surface tensiondriven. In this way, the interfacial thermal transfer structure 102enables the flow of excess, e.g., paste, away from areas of nearestcontact to areas that benefit from additional paste or grease. Theeffect is analogous to dipping a bundle of capillary tubes of differingdiameter in a fluid, i.e., capillary action. Due to capillary action,the smallest diameter tubes have the highest capillary driving force andthus, fill first and to a higher level than the larger diameter tubes.The presence of capillary pores may enable the interfacial thermaltransfer structure 102 to maintain a minimum BLT throughout the life ofthe interfacial thermal transfer structure 102, and the heat sink 104.The interfacial thermal transfer structure 102 also enables a method tomanage non-coplanarity or warpage (static and dynamic) issues for largeIHS packages. Non-coplanarity issues involve transferring heat betweencontacting bodies that are curved or out of flatness. Depending on thetype interface material, non-coplanarity might induce large air gaps orlarge bondline thickness. Static and dynamic warp issues refer to havingout-of-flatness at room temperature (or a steady-state), and transientvariation in flatness due to temperature cycling, respectively. Staticwarp may be related to the manufacturing process inducing a concave,convex, wave, or non-flat profile on a surface. Dynamic warp may be aconcave, convex, wave, or non-flat profile on a surface due to thermalcontraction and expansion of the material, or a change in the surfaceprofile when subjected to a force or load.

A potential mechanism for voiding is via paste pumping during repeatedthermal and pressure cycles by trapping air cells. While the physics ofvoiding is not completely understood, it is known that pressuregradients, viscosity and surface tension play a role. The interfacialthermal transfer structure 102 creates a pathway to reduce the pump outof pastes or greases during the repeated thermal cycling by reducing thepressure gradients in comparison to conventional TIMs alone. Thepressure reduction is achieved by provision of smaller capillary flowchannels in the imperfections of the in-contact surfaces of thestructure 102, the heatsink 104, and the source (not shown), thatenables liquid flow along the mesh wires of the interfacial thermaltransfer structure 102 and the larger pore 108 networks that enablelong-range evacuation or volume-filling of paste.

In contrast, an interface with conventional TIMs, but not theinterfacial thermal transfer structure 102, does not enable the thermalinterface materials to flow between induced pressure gradients duringthermal cycling. Instead, voids form and grow over time. Accordingly,providing an interface that also includes the interfacial thermaltransfer structure 102 allows for a reduction in flow resistance betweenregions of higher and lower pressure. This slows the formation andgrowth of voids over time.

In one application using IHS specifications, commercial thermal grease(kpaste=3.5 W/mK) as TIM2, and off-the-shelf copper (kCopper=390 W/mK)wire screen mesh as wick, estimated effective thermal conductivity was11.9 W/mK for 90% porosity wick. Effective thermal conductivity wasdetermined using the relationship keff=kpaste(kCopper/kpaste)(1−ε)̂0.59where c is the porosity of the screen wick. This increase in thermalconductivity translates (after accounting for contact resistance andwarpage contribution) approximately to 200-300 percent reduction in TIM2thermal resistance (for End of Life requirements).

FIG. 1B is a block diagram showing the interfacial thermal transferstructure 102 integrated to the heat sink 104. FIG. 1B showsmeasurements of the various components of the structure 102 and heatsink 104.

FIG. 2A is a block diagram showing interfacial thermal transferstructures 202-1, 202-2 integrated to a heat sink 204. In an embodiment,screen meshes can be stacked on top of one other, either aligned oroffset, to increase effective thermal conductivity and the thickness ofthe thermal interface material. In FIGS. 2A, 2B, interfacial thermaltransfer structure 202-2 is shown stacked on top of interfacial thermaltransfer structure 202-1, in an offset position. FIG. 2B is a blockdiagram showing a side view of interfacial thermal transfer structures202-1, 202-2 integrated to heat sink 204.

In an alternative embodiment, wicking patterns or capillary flow pathscan be directly printed on the base of the heat sink itself (or anycontacting body) by conventional processes, such as machining, embossingand so on. FIG. 3A is a block diagram showing capillary trenches 302-Aprinted on a heat sink 304-A. FIG. 3B is a block diagram showingcapillary trenches 302-B printed on a heat sink 304-B.

In another embodiment, interfacial thermal transfer structures can bestrategically placed for an improvement in thermal transfer in specificareas. For example, FIG. 4 is a block diagram showing an interfacialthermal transfer structure 402 integrated to a heat sink 404.

FIG. 5 is a block diagram showing an embodiment of an interfacialthermal transfer structure 500. The interfacial thermal transferstructure 102 can be used in other ways to enable a pathway forcontrolled dispersal of high thermal conductivity particles. Usingsolution synthesis or dip coating methods, conductive particles 502 canbe placed on a structure 504, a priori to integration with the heat sink104. These conductive particles 502 may be attached to surfaces of thestructure 504 by Van der Waals forces. During squeeze flow (ofpolymers), the particles “fall” off the structure 504 to enablelocalized dispersion of conductive particles 502. After squeeze flow ofthermal paste (with lower volume fraction particles, and hence lowerviscosity) particles 502 are locally dispersed.

FIGS. 6A-3B are block diagrams showing an example material 600 of aninterfacial thermal transfer structure. In one approach for designingporous materials for the interfacial thermal transfer structure 102, thepores of the material 600 of the structure 102 can be geometricallyconfigured to enable negative Poisson's ratio behavior. As understood byone of ordinary skill in the art, Negative Poisson ratio materials swellwhen stretched and get thinner when compressed. In FIG. 6A, an examplenegative Poisson ratio material 600 is shown swollen. In FIG. 6B, thematerial 600 is in a deformed state of negative Poisson ratio materialdue to tensile forces. This design of pores is anticipated to mitigatecoefficient of thermal expansion (CTE) mismatch related thermalstresses. It is noted that the negative Poisson ratio material isexpanding laterally when pulled by tensile forces as against contraction(in the lateral dimensions) in a conventional positive Poisson ratiomaterial.

FIG. 7 is a block diagram of a computing device 700 including aninterfacial thermal transfer structure. The computing device 700 may be,for example, a laptop computer, desktop computer, tablet computer,mobile device, or server, among others. The computing device 700 mayinclude a central processing unit (CPU) 702 that is configured toexecute stored instructions, as well as a memory device 704 that storesinstructions that are executable by the CPU 702. The CPU may be coupledto the memory device 704 by a bus 706. Additionally, the CPU 702 can bea single core processor, a multi-core processor, a computing cluster, orany number of other configurations. Furthermore, the computing device700 may include more than one CPU 702. The instructions that areexecuted by the CPU 702 may be used to implement shared virtual memory.The memory device 704 can include random access memory (RAM), read onlymemory (ROM), flash memory, or any other suitable memory systems. Forexample, the memory device 704 may include dynamic random access memory(DRAM).

The CPU 702 may also be linked through the bus 706 to a displayinterface 708 configured to connect the computing device 700 to adisplay device 710. The display device 710 may include a display screenthat is a built-in component of the computing device 700. The displaydevice 710 may also include a computer monitor, television, orprojector, among others, that is externally connected to the computingdevice 700.

The computing device also includes a storage device 712. The storagedevice 712 is a physical memory such as a hard drive, an optical drive,a thumbdrive, an array of drives, or any combinations thereof. Thestorage device 712 may also include remote storage drives. The storagedevice 712 includes any number of applications 714 that are configuredto run on the computing device 700.

The computing device 700 may also include a network interface controller(NIC) 716 may be configured to connect the computing device 700 throughthe bus 706 to a network 718. The network 718 may be a wide area network(WAN), local area network (LAN), or the Internet, among others.

According to embodiments described herein, the computing device 700 alsoincludes an interfacial thermal transfer structure 720 and a heat sink722. The interfacial thermal transfer structure 720

The block diagram of FIG. 7 is not intended to indicate that thecomputing device 700 is to include all of the components shown in FIG.7. Further, the computing device 700 may include any number ofadditional components not shown in FIG. 7, depending on the details ofthe specific implementation.

All optional features of the computing device described above may alsobe implemented with respect to either of the methods described herein ora computer-readable medium. Furthermore, although flow diagrams andstate diagrams may have been used to describe embodiments, the presenttechniques are not limited to those diagrams or to the correspondingdescriptions herein. For example, flow need not move through eachillustrated box or state or in exactly the same order as illustrated anddescribed.

The present techniques are not restricted to the particular detailslisted herein. Indeed, those skilled in the art having the benefit ofthis disclosure will appreciate that many other variations from theforegoing description and drawings may be made within the scope of thepresent techniques. Accordingly, it is the following claims includingany amendments thereto that define the scope of the present techniques.

What is claimed is:
 1. An apparatus that interfaces thermal transfercomponents, the apparatus comprising a soft, thermally conductive metalthat enables a capillary flow path with a contact surface of a thermaltransfer component comprising a heat sink, and an imbibing thermalinterface material, the thermally conductive metal comprising largepores that intertwine with smaller pores of the contact surface.
 2. Theapparatus of claim 1, comprising expanded metal grids that facilitatethe creation of a dual-porosity wick.
 3. The apparatus of claim 2,wherein the dual-porosity wick comprises a woven wire mesh.
 4. Theapparatus of claim 1, being integrated with a base of the heat sink. 5.The apparatus of claim 1, comprising a thickness ranging between 75 μmand 150 μm.
 6. The apparatus of claim 1, the large pores comprisinginterstitial spaces comprising the imbibing thermal interface material.7. The apparatus of claim 1, the larger pores ranging in diameter from 1mm-2 mm.
 8. The apparatus of claim 1, the smaller pores ranging indiameter from 20 μm-100 μm.
 9. The apparatus of claim 1, the capillaryflow enabling a flow of excess imbibing thermal interface material fromareas of nearest contact between the apparatus and the heat sink, toareas that benefit from additional imbibing thermal interface material.10. An apparatus that interfaces thermal transfer components, theapparatus integrated with a base of a heat sink, the apparatuscomprising a soft, thermally conductive metal that enables a capillaryflow path with a contact surface of a thermal transfer componentcomprising a heat spreader and an imbibing thermal interface material,the metal thermally conductive metal comprising large pores thatintertwine with smaller pores of the contact surface.
 11. The apparatusof claim 10, comprising a printed capillary flow path on the contactsurface.
 12. The apparatus of claim 11, the printed capillary flow pathgenerating expanded metal grids that facilitate the creation of adual-porosity wick.
 13. The apparatus of claim 10, comprising aplurality of thermally conductive mesh.
 14. The apparatus of claim 11,comprising a plurality of thermally conductive mesh.
 15. The apparatusof claim 11, the plurality of thermally conductive mesh comprising largepores overlaying with respect to large pores of other mesh.
 16. Theapparatus of claim 11, comprising a plurality of thermally conductivemesh with large pores interspaced with respect to large pores of othermesh.
 17. The apparatus of claim 10, large pores comprising interstitialspaces comprising the imbibing thermal interface material.
 18. Anapparatus that interfaces a heat sink and a heat spreader, the apparatuscomprising a soft, thermally conductive metal that enables a capillaryflow path with a contact surface of the heat sink, a contact surface ofthe heat spreader and an imbibing thermal interface material, when theapparatus is in contact with a portion of a surface of both the heatsink and the heat spreader, the thermally conductive metal comprisinglarge pores that intertwine with smaller pores of the contact surface,the large pores comprising interstitial spaces comprising the imbibingthermal interface material.
 19. The apparatus of claim 18, comprising aprinted capillary flow path on the contact surface.
 20. The apparatus ofclaim 19, the printed capillary flow path generating expanded metalgrids that facilitate the creation of a dual-porosity wick.
 21. Theapparatus of claim 20, comprising a plurality of thermally conductivemesh.
 22. The apparatus of claim 21, the plurality of thermallyconductive mesh comprising large pores interspaced with respect to largepores of other mesh.